This invention relates to the art of nuclear reactors and has particular relationship to the upeer-internals structure of nuclear reactors. A nuclear reactor includes a pressure vessel into which a heat-transfer fluid, typically liquid sodium for fast breeder reactors, or pressurized or boiling water for more conventional commercial reactors, is pumped under pressure. The fluid flows through the core and is heated; the hot fluid emerges from the vessel and the heat flows via mechanically separated primary and secondary loops to electrical-power generating equipment. Within the vessel there is supporting structure for the core components. Typically, for a liquid-cooled fast breeder nuclear reactor which generates more fissile fuel than it burns up, these components include fuel-rod bundles or assemblies, control-rod assemblies, blanket fertile-material or fertile-rod assemblies and removable radial shielding assemblies. The expression "core assemblies" or "core component assemblies" or the word "assembly," when used in this application with reference to components of the core, means one or more types of these assemblies. The core-support structure serves the purposes of locating, supporting, distributing coolant to, and providing axial and radial restraint for, these assemblies.
The core component assemblies, which in the illustrated embodiment include fuel assemblies, of both fissile and fertile fuel-containing types, control-rod assemblies and shielding assemblies, which form the core of a liquid metal-cooled fast-breeder nuclear reactor, are separately supported in inlet-support modules or modular units. Each inlet-support modular unit is removably mounted, held only by gravity, in liners in the lower core-support structure with fluid seals interposed between the aligned fluid inlet openings in the module and liner and the upper and lower parts of the module and liner. Each module directs flow of the heat-transfer or coolant fluid to a plurality (typically 7) of reactor component assemblies which are removably mounted, held only by gravity, in receptacles of the corresponding modular unit. Below the seal each module is subjected to low pressure which balances the low pressure in the region where the fluid emerges from the core components. The low pressure in the volume below the module lower seal is generated and maintained by venting this volume to the low pressure regions of the vessel of the reactor. Gravity is adequate to hold the modules in the liner.
Typically, this invention applies to a 975 Megawatts-thermal (Mwt) 400 Mwt.-electrical (Mwe) liquid-metal cooled fast-breeder reactor which has 198 hexagonal-core fuel assemblies surrounded by 150 radial blanket assemblies and 324 radial shield assemblies. In this typical reactor the assemblies are received in 61 inlet modules each having 7 receptacles. The velocity of the heat-transfer or cooling fluid, which is sodium, and its distribution varies with the character of the component or assembly which it cools. The velocity is about 30 feet per second in non-replaceable components while in replaceable components it may be as high as 50 feet per second at the inlet-lower-temperature end and 40 feet per second at the outlet-higher-temperature end. In the fuel rod bundles or assemblies it is 25 feet per second. Eighty percent of the fluid is allocated to the core, 12% to the radial blanket, 1.6% to control assemblies, and the remainder to shielding, bypass and leakage.
Typically a reactor of the type to which this invention relates, for example, a sodium-cooled breeder reactor, operates at a bulk coolant temperature differential of 300.degree. F or greater, between the core inlet and core outlet. This temperature gradient is not uniform across the core; it fluctuates widely and has one major peak in temperature across the core caused by core geometry. Localized temperature variations may also occur by reason of local anomalies in the core such as fuel "burnup," deliberate variations in fuel enrichment, and control assemblies. Also, typically, a sodium-cooled breeder reactor undergoes rapid and severe changes in the core outlet temperature because of rapid changes in power load-level during postulated `upset` events such as reactor trips, rapid unloading, etc.
The structure within the reactor vessel above the core, variously called instrument trees, upper-core support structures, or upper-internals structure, or upper internals as it is called in this application, provides primary or secondary `holdown` of the reactor core for the contingency that the gravity holddown fails due to loss of the low pressure `balance` as explained above during emergencies such as scram and also supports the control-rod drivelines and instrumentation. These upper internals are exposed to the core effluent flow, thermal gradients, thermal transient conditions and periodic "stripping" of hot and cold coolant streams. The word "stripping" means the overlap in temperature which occurs between adjacent parts of a reactor, for example adjacent core-component assemblies, which operate at widely different temperatures. The resulting thermal stress and thermal fatigue may reduce the design lifetime of upper-internals structures, which are normally designed for a lifetime equal to that of the reactor itself.
In accordance with the teachings of the prior art an attempt has been made to mitigate the effects of the stresses produced by the sharp differences and fluctuations in temperature by providing the upper internals, typically of a sodium-cooled breeder reactor, with a massive plate or structures (typically about 5 tons in weight) which serves both as holddown for the core-component assemblies and also to transmit the coolant from the outlet of the core. This plate has separate openings for the effluent from each of the core-component assemblies. Each opening has a thermal liner or sleeve whose purpose is to mitigate transient rates or changes in temperature. This prior-art structure is not satisfactory.
It is an object of this invention to provide a nuclear reactor having upper internals which shall effectively mitigate the thermal stress and thermal fatigue resulting from the temperature differences and temperature fluctuations of the effluent from the outlet of the core and shall assure a lifetime of the upper internals at least equal to the lifetime of the reactor itself.